Type 1 diabetes (T1D) is one of the most common chronic childhood diseases requiring life-long insulin therapy. Children and adolescents with T1D need regular insulin injections or the continuous insulin delivery using an insulin pump in order to keep blood glucose levels normal. The investigators know that keeping blood sugars in the normal range will help prevent long-term diabetes-related complications involving the eyes, kidneys and heart. However, achieving treatment goals can be very difficult as the tighter the investigators try to control blood glucose levels, the greater the risk to develop symptoms and signs of low glucose levels (hypoglycaemia). This is a particular problem at night and one solution is to develop a system whereby the amount of insulin injected is controlled by a computer and is very closely matched to the blood sugar levels on a continuous basis. This can be achieved by what is known as a "closed-loop system" where a small glucose sensor placed under the skin communicates with a computer containing an algorithm that drives an insulin pump. The investigators have been testing such a system in Cambridge over the last three years in children and have found that this system is effective at preventing nocturnal hypoglycaemia. The next stage of this research is to test the system for a longer period of time at home. In the present study the investigators are planning to study 16 adolescents aged 12-18 years on insulin pump therapy. During 21 nights glucose will be controlled by the computer and during the other 21 nights the subjects will make their own adjustments to the insulin therapy. The investigators will then analyze the data to determine the effect of the computer algorithm in keeping glucose levels between 3.9 and 8 mmol/L (normal levels) and reducing the time they spent with glucose below 3.9 mmol/L (hypoglycaemia). Subjects' response to the use of the system in terms of life-style change, daily diabetes management and fear of hypoglycaemia will be assessed. The investigators will also test for longer term glucose control by measuring glycated haemoglobin and other blood parameters.